The invention relates to a method for sintering nuclear fuel in a microwave furnace and to a corresponding microwave furnace.
The industrial use of microwave furnaces is currently restricted to the drying of bodies or materials, sterilizing (for example of foodstuffs), the polymerizing of rubber, curing of plastics and similar processes which take place at moderate temperatures. The ceramics industry is interested in the use of microwaves for sintering, but that has previously been restricted to virtually only a laboratory scale. That is because although past experience shows that shorter sintering periods are adequate, higher temperatures are allegedly necessary (causing greater wear on the furnaces) and altogether higher energy losses occur, although better material properties (for example a finer grain in the ceramic structure) can possibly be achieved. However, until now no products with a satisfactory quality have been achieved at all with microwaves.
However, in International Application No. PCT/EP97/04513, which is not a prior art publication, there is a description of a process by which green compacts that are pressed from unsintered nuclear fuel are sintered to form finished sintered nuclear-fuel compacts. In that case, not only the shape of the sintered bodies and their density but also the mechanical/chemical properties meet the requirements for use in nuclear reactors. In that case, over the same period of time, lower temperatures are necessary than in conventional processes, with the result that maintenance is simplified and that wear and energy losses are reduced. However, the configuration described therein, which is constructed on an empirical basis, is difficult to optimize. The aimed-for homogeneous temperature distribution in the fuel, low temperature losses and low thermal stressing of the furnace parts are difficult to achieve and not always reproducible.
The special characteristic of ceramic nuclear fuel is that it xe2x80x9ccouplesxe2x80x9d adequately well to the microwaves, i.e. it can absorb energy from the microwave field, without being electrically conductive at low temperatures. However, at higher temperatures the electrical conductivity increases and the fuel increasingly behaves like a metal. Local overheating, arcs and distortions of the microwave field therefore occur (for example, an already well-sintered, conductive region may hinder the microwaves from penetrating into neighboring regions of the fuel). That results in irregularly sintered, partially melted and deformed pellets. Therefore, the aim is to achieve the most homogeneous possible distribution of the energy and temperature without highly pronounced local maxima.
According to that older proposal, the microwaves are generated by a magnetron or a similar electrical component (for example a klystron) and passed through a waveguide into a furnace chamber (working chamber), which is constructed as a resonator, i.e. it is shielded on all sides by microwave-reflecting (metallic) walls. In that case, the magnetron is regarded as the sole source of the microwave field, the nuclear energy is regarded as just a sink of the field and the waveguides with the resonance chamber are regarded merely as a lossy transmission of the microwaves. It is intended for the geometry of the resonance chamber and of the waveguides to be empirically chosen in such a way that the heat losses are minimized. In other words, as much energy as possible is taken from the field by the nuclear fuel. In addition, by changing the position of the microwaves at the working chamber, the most uniform possible temperature distribution is set in the fuel. In order to provide the necessary power, a plurality of magnetrons are respectively provided over a waveguide, which has one end that merges with its full cross section into the resonance chamber. The individual magnetrons are individually controlled, in order to bring about the most homogeneous possible temperature distribution by superposing the wave fields generated by them.
Uniform quality is achieved in that case only by pushing the material to be sintered through a ceramic tube which has sintering gas flowing through it and extends transversely through the entire resonance chamber. With the unavoidable local inhomogeneities of the wave field and the temperature distribution, all of the regions of the fuel are then subject to the same local conditions, so that ultimately all samples of the fuel should have the same prehistory with regard to the temperatures they have undergone. A precondition therefor is that the microwave field does not undergo any pronounced fluctuations over time. With regard to the temperatures, sintering times, sintering atmospheres and. advantageous devices provided for the sintering (for example gas locks for introducing the fuel into the tube through which the sintering gas flows) and further details of a sintering system with microwaves, that document contains a wealth of proposals which can also be applied to the present invention. The content of that document therefore also constitutes part of the content of the present application, with which the radiating of the microwaves into the working chamber (resonance chamber) is improved.
It is accordingly an object of the invention to provide a method and a microwave furnace for the sintering of microwave fuel of a quality required for use in a reactor, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for treating nuclear fuel in a microwave furnace, which comprises feeding microwaves from a microwave radiator into an antenna cavity; feeding the microwaves from the antenna cavity through a plurality of narrow connecting openings into a resonance chamber; and introducing nuclear fuel into the resonance chamber.
With the objects of the invention in view, there is also provided a microwave furnace for treating nuclear fuel at temperatures of between 20 and 2000xc2x0 C. and an average temperature of between 1200 and 1800xc2x0 C., comprising a resonance chamber shielded on all sides by walls reflecting microwaves; a gassing and degassing system associated with the resonance chamber; at least one holder for nuclear fuel in the resonance chamber; an access for introducing nuclear fuel into and removing nuclear fuel from the resonance chamber; an antenna cavity shielded on all sides by walls reflecting microwaves; a separating wall separating the antenna cavity from the resonance chamber, the separating wall having at least one narrow opening formed therein providing an interconnection between the antenna cavity and the resonance chamber; and at least one microwave radiator disposed outside the resonance chamber and feeding into the antenna cavity.
With the objects of the invention in view, there is additionally provided a microwave furnace for producing sintered nuclear-fuel compacts by sintering molded green compacts of nuclear fuel in a sintering gas at average temperatures of between 1200 and 1800xc2x0 C., comprising an elongate resonance chamber shielded on all sides by walls reflecting microwaves, the resonance chamber having a longitudinal side and a longitudinal direction; a gassing and degassing system associated with the resonance chamber; at least one elongate holder associated with the resonance chamber for holding green compacts; an access associated with the resonance chamber for introduction and removal of green compacts; an elongate antenna cavity shielded on all sides by walls reflecting microwaves; a separating wall separating the antenna cavity from the resonance chamber; the antenna cavity connected to the resonance chamber by a plurality of slots mutually offset in the longitudinal direction of the resonance chamber; at least one waveguide on the longitudinal side of the resonance chamber, the waveguide having an open end leading into the antenna cavity and an opposite closed end; and a microwave radiator disposed at the closed end of the waveguide.
xe2x80x9cNuclear fuelxe2x80x9d is understood herein to mean not only uranium oxide itself but also mixtures with other oxides (in particular transuranic elements such as plutonium and thorium) as well as absorber materials (such as gadolinium oxide). The invention is preferably intended for the sintering of pressed shaped bodies of the fuel (so-called xe2x80x9cgreen compactsxe2x80x9d) to form corresponding sintered compacts (generally cylindrical xe2x80x9cpelletsxe2x80x9d). However, it is also suitable for handling powder or granules at corresponding sintering temperatures. This is because experience with the present invention shows that, at least with the materials suitable for use in a nuclear reactor, a reduction of the sintering temperatures and of the heat losses can be expected from the use of microwaves.
The invention is based on the assumption that the increasing electrical conductivity of the nuclear fuel at high temperatures not only leads to deteriorations in the sintering result but also to unstable conditions in the field. The reason for this is that, in accordance with the way in which it is inhomogeneously heated up, the fuel not only becomes an inhomogeneous sink for the microwave radiation but acts itself in a way similar to a xe2x80x9ctransmitterxe2x80x9d, due to the electrical properties, so that unstable feedback to the magnetrons and the radiation emitted by them occurs. This feedback cannot be reliably handled with a configuration which treats the furnace chamber (resonance chamber) merely as a device for transferring the radiation between the magnetron (source) and the nuclear fuel (absorber) that is to be optimized with regard to losses.
Rather, in the first instance the invention uses an antenna cavity which is closed on all sides through the use of (metallic) walls reflecting microwaves and is matched in its dimensions to the microwave radiation being used, in order to produce a stable microwave field (stationary wave). Usually, a magnetron or a klystron is used in microwave technology to generate a frequency of 915 MHz or 2.45 GHz. Generally, a frequency of between 0.4 and 30 GHz is suitable. The dimensions of low-loss waveguides tuned to these frequencies have been investigated, are known and have been described. If these waveguides are closed off at the ends by reflecting walls (so-called xe2x80x9cshorting terminationsxe2x80x9d), they become resonators, in which such frequencies lead to stationary wave fields.
According to the invention, each antenna cavity of this type is preferably assigned an individual magnetron (or klystron). However, the magnetron is not disposed within the stationary wave but is located at the end of a corresponding waveguide, which opens into the antenna cavity with its other end.
The energy required for the sintering is extracted from the field stabilized in this way in the antenna cavity, through a plurality of narrow openings in a wall of the antenna cavity and radiated into the resonance chamber. In comparison with the surface area of a wall of the antenna cavity, these openings, which are preferably formed as slots, are so small that they have virtually no influence on the formation of the stationary wave in the antenna cavity, do not induce any electrical sparkovers, but emit sufficient power. As a result, the feedback of microwave radiation into the antenna cavity is also minimized. Such xe2x80x9cslot antennasxe2x80x9d have previously already been proposed for communications technology, in order to emit corresponding fields in virtually an infinite environment from which only slight reflections are returned. This is intended to produce a stabilized uniplanar radiator with a radiant power distributed uniformly over the surface area of the radiator.
The technology of such a slot antenna is described in a paper entitled xe2x80x9cA Multislotted Waveguide Antenna for High-Powered Microwave Heating Systemsxe2x80x9d by Werner Rxc3xcggeberg in IEEE Transactions on Industry Applications, Vol. IA-16, No. 6, November/December 1998, pages 809 to 813. Described in the paper are procedures and formulas with which the emitted power as well as the number and configuration of the slots are determined, in order to obtain a desired uniplanar distribution of the radiant power. In that case, the irradiated material is regarded as an infinite space in which considerable energy is emitted but the emitted energy is not reflected. Therefore, only low temperatures are observed at the location of the absorber as well. If, however, a metallic body which could simulate nuclear fuel heated to a high temperature with respect to reflection and absorption is brought from outside into the vicinity of the slots, there is a breakdown of the stationary wave in the antenna cavity in which, according to Ruggeberg, the magnetron is also disposed. Arcs occur along with considerable damage to the walls and the magnetron of the antenna and to the reflector. When such a slot antenna according to Rxc3xcggeberg was fitted into the configuration according to International Application No. PCT/EP97/04513, that damage occurred even though the antenna power was restricted and the average temperature in the nuclear fuel still did not reach sintering temperature.
Nevertheless, the invention provides an antenna cavity which is fed by a microwave radiator and has a wall with at least one narrow opening (advantageously: a plurality of slots) for the coupling of microwaves into a resonance chamber contained in the nuclear fuel. However, the slots are constructed in such a way that the feedback to the antenna cavity by reflections at the nuclear fuel is no longer disruptive. Rather, the changed configuration of the slots can allow the temperature distribution in the nuclear fuel to be controlled and set.
According to the invention, a plurality of such slot antennas are advantageously used, in order to couple the energy necessary for sintering the fuel into the resonance chamber. The resonance chamber advantageously has the same length as the antenna cavity and the antenna cavity is disposed on one longitudinal side of the resonance chamber. In the simplest case, the antenna cavity is situated directly at the resonance chamber, so that the two chambers are separated by a common wall, which has the narrow openings or slots.
If the already mentioned metallic behavior of highly heated fuel is taken into consideration, the material to be sintered consequently becomes not only a strong absorber, but a xe2x80x9ctransmitterxe2x80x9d (or least a reflector), the feedback of which to the stationary wave in the antenna cavity must not be ignored. Rather, the resonance conditions in the antenna cavity are greatly detuned by the feedback with the nuclear fuel.
If it is desired to avoid this feedback, the number and/or surface area of the slots could be reduced. That has the effect of reducing the energy being fed back, but the energy absorbed by the nuclear fuel and required for the sintering is also reduced, so that in practice the fuel is not heated to the necessary sintering temperature. That approach is consequently not feasible.
Rather, the system must be regarded as a feedback system and the antenna cavity with the slots must be constructed from the outset for the detuned conditions. This is possible empirically in a simple way by changing the length of the antenna cavity through the use of displaceable metallic terminations and by varying the position of the slots in the walls of the antenna cavity. It is found that, by changes of this type, a significantly more homogeneous distribution of the temperature and field in the resonance chamber can be achieved and the damage mentioned above can be avoided.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a furnace for microwave sintering of nuclear fuel, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.